Fetal studies show that a specific malformation, e.g., polymicrogyria or cobblestone cortex, may be the end point of disruption of one of a number of developmental pathways and may have more than one etiology. Histology of the cerebral hemispheres showed, apart from necrosis, failure of neuronal migration with subcortical heterotopia, polymicrogyria, overmigration into the leptomeninges, transmantle dysplasia, and schizencephaly. Malformations resulting from undermigration may be generalized, as in type 1 lissencephaly and subcortical band (double cortex) syndromes, or focal as represented by subcortical and periventricular heterotopias. Focal cortical dysplasia and tuberous sclerosis are lesions restricted to a small region of the cortex and are thought to result from early abnormalities in cell growth and proliferation. Cobblestone cortex (type 2 lissencephaly), bilateral frontoparietal polymicrogyria, MARCKS deficiency, TUBB2B mutation, polymicrogyria and schizencephaly are some of the over migration syndromes.